Activated carbon is used in various gas cleaning applications, and is finding increasing application in mercury control in flue gas from coal-fired power plants and waste incinerators. In a typical application, powdered activated carbon (d50—particle size corresponding to 50% of the mass of the sample—approximately 25 microns) is injected into the flue gas duct upstream of a particulate collection device such as a precipitator, fabric filter, cyclone or particulate scrubber. The injected powdered activated carbon particles capture mercury species from the flue gas and are removed in the collection device. In this manner, the mercury species are prevented from being emitted into the environment via the flue gas. If the particulate collection device is a precipitator then the mercury removal efficiency for a given sorbent consumption rate is typically significantly lower than when the collection device is a fabric filter. This is because the fabric filter allows significantly improved contact between the flue gas and the sorbent when the sorbent is collected on the bag compared to a precipitator, where the flue gas-sorbent content happens only while the sorbent particles are suspended in the flue gas.
The injection location for the activated carbon in the power plant can vary. Activated carbon has been injected at lower temperatures of around 300° F. into the flue gas to capture mercury as well as at higher temperatures of around 600 to 800° F. (See U.S. Pat. Nos. 6,848,374 and 6,953,494). Typical carbon injection rates that are considered economical vary between 0.2 to 1.0 lb/MWh.
Powdered activated carbon can be injected into the flue gas in a coal-fired power plant at several locations. One such location is the region upstream of the air heater at temperatures between 500 and 900° F. Powdered activated carbon is typically injected with air as the carrying medium and through lances that penetrate completely into the flue gas duct to ensure good distribution of the sorbent with the flue gas.
During the process of injection at high temperature locations such as into the flue gases at the air heater inlet in a coal-fired plant, the carbon particles are subjected to high temperatures as well as high oxygen concentrations within the injection lances. Consequently, the carbon particles can start oxidizing and burning within the injection lances, deteriorating the sorbent and potentially even destroying it, as well as potentially causing other operational problems such as local deposition, plugging and over-heating. This is one additional challenge of using activated carbon sorbent for mercury control.
When powdered activated carbon is used in various applications with differing flue gas compositions, the efficiency of mercury capture varies significantly. For example, in the case when coal with low levels of halogen is combusted, the flue gases generated therein have a low concentration of halogen species such as HCl. In such cases, plain activated carbon performs poorly, i.e. the amount of material required to achieve desired capture efficiency is higher than when higher levels of halogen species are present in the flue gas. Methods to overcome this problem have been described, such as in U.S. Pat. No. 6,953,494, and include adding a halogen component to the activated carbon sorbent prior to injection in the flue gas. A method to add the halogen component as a separate stream from the activated carbon has also been used to address this issue. The literature shows data on the improved performance of halogen-treated carbon for low-halogen content flue gas applications versus a plain activated carbon.
U.S. Pat. No. 4,500,327 describes a method of removal of mercury vapor and an adsorbent for the same, comprising activated carbon having as supported thereon one or more components including sulfur, sulfates and nitrates of various metals or ammonium, and bromides and iodides of K, Na or ammonium, oxide of iodine, oxy-acid corresponding to oxide of iodine or salt of the said oxy-acid.
U.S. Pat. No. 5,556,447 describes a process for mercury capture wherein a carbonaceous sorbent doped with a halide salt is used to remove mercury from process gases containing mercury vapor. In the same patent a process for removing mercury is also described wherein the carbonaceous sorbent is injected in the form of an aqueous slurry, where the aqueous slurry further comprises ammonia, ammonium chloride, ammonium hydroxide, or sodium hydroxide.
U.S. Pat. No. 6,953,494 describes preparing a mercury sorbent by treating a carbonaceous substrate with an effective amount of a bromine-containing gas, especially one containing elemental bromine or hydrogen bromide, for a time sufficient to increase the ability of the carbonaceous substrate to adsorb mercury and mercury-containing compounds. The locations in the flue gas stream of injecting and collecting and removing the mercury sorbent may be varied, depending upon the exact configuration of the exhaust gas system.
U.S. Pat. No. 6,848,374 describes the deposition of a halogen on activated carbon by vaporizing halogens and condensing/adsorbing on the carbon sorbent. The patent describes another method of spraying a solution of halide salt and then delivering the wet sorbent to a jet mill/particle-particle separator. The patent does not describe a method of simultaneously drying and reducing the particle size of the carbon sorbent. Also, the need to perform particle size reduction in situ involves adding equipment for a particle size reduction system such as a jet mill (and compressed air), thus making the sorbent storage and delivery system more complicated to operate, and more expensive. In contrast, if pre-ground fine particles can be delivered, and they remain in a state that will allow their re-dispersion, such storage and delivery systems would not be needed. Also, the patent describes injection of a SO3 sorbent separately from the mercury sorbent, but does not recognize the benefit of association of the alkaline material with the halogen-loaded carbon sorbent for mercury control.
U.S. Pat. Nos. 6,638,347 and 6,524,371 disclose powdered sorbent and method for removing mercury, other metals, and contaminants from a gas stream comprising powder that is characterized as containing a carbon-based powder selected from the group consisting of coal carbons, wood carbons, graphite carbons, activated carbons, coconut shell carbons, peat carbons, petroleum cokes, synthetic polymers, the like, and combinations thereof, and an effective amount (about 3 to about 10 weight percent) of cupric chloride. Optionally, sulfur, potassium iodide and permanganate, calcium hydroxide, and combinations thereof may be added to the powder. However, this composition has only been tested with fabric filters as the particulate collection device, and may not provide similarly good results when a precipitator is used for removing particulates. Also, the patents do not describe any details of how, for example, calcium hydroxide is added to the adsorbent powder.
US patent application publication 20060210463 describes processes and compositions for decreasing emissions of mercury upon combustion of fuels such as coal. Various sorbent compositions are provided that contain components that reduce the level of mercury and/or sulfur emitted into the atmosphere upon burning of coal. In various embodiments, the sorbent compositions are added directly to the fuel before combustion; are added partially to the fuel before combustion and partially into the flue gas post combustion zone; or are added completely into the flue gas post combustion zone. In preferred embodiments, the sorbent compositions comprise a source of halogen and preferably a source of calcium. Among the halogens, iodine and bromine are preferred. In various embodiments, inorganic bromides make up a part of the sorbent compositions. In this case, no carbonaceous sorbent is used, and the different components halogen and source of calcium (e.g. lime) are injected separately.
U.S. Pat. No. 6,974,564 discloses a clay and limestone byproduct from the papermaking industry (MinPlus™) as an adsorbent for mercury, which is injected into the high temperature region flue gas (around 2000° F.). The injection rates for 95 percent capture are in the range of 20 lb/MWh, a factor of 20 larger than typical activated carbon injection and 0.8-3.2 lb/MWh, for 75 percent reduction, a factor of 4 larger than typical carbon injection rates. The high injection rates are likely to be expensive, and also may affect the performance of the power plant components by depositing and fouling their surfaces. Also the injection of large quantities of material may adversely affect the performance of particulate collection devices and may result in increased particulate emissions.
A problem with many mercury control sorbents is their inability to perform with high efficiency in high concentration sulfur-containing flue gas. It has been determined that halogen-treated activated carbons for mercury control in relatively high sulfur concentration flue gases must be used at significantly higher quantities (almost 5 to 10 times) in order to achieve large reductions in mercury emissions compared to cases where the flue gases had low concentration of sulfur species such as sulfur dioxide and sulfur trioxide. The coal sulfur concentration was 2.5 percent in the high sulfur case compared to about 0.4 percent sulfur in the coal in the low sulfur case. It has been shown that a lack of halogen components in the flue gas and the presence of sulfur species in the flue gas each adversely affect the performance of activated carbon in terms of mercury removal.
Yet another problem with using activated carbon sorbents for mercury control is that when the spent carbon is mixed with the ash in the particulate collection device, it renders the ash unusable for some end-applications such as concrete. This is because when fly ash is used in concrete manufacturing, if it has certain components that adsorb the hydrophobic air entraining agents that are used in concrete manufacturing, then it is rejected for such end use. Activated carbon because of its surface area and its propensity to adsorb the air entraining agents is deleterious. A foaming index test is used to evaluate the suitability of the ash for use in concrete. If the foaming index is below a critical value, then the ash is suitable for use in concrete.
Methods to decrease the propensity of carbon from adsorbing air entraining agents have been disclosed in US Patent Application publication 20030206843. These involve modifying the surface chemistry of the carbon by treating with oxidizing components such as ozone. However, treatment with ozone is expensive and is an additional step in the sorbent preparation process. Also, the performance of the ozone-treated carbon for mercury capture is significantly deteriorated compared to the untreated carbon.
US Patent Application publication 2004/0206276 discloses the use of “sacrificial agents” that are added to cementitious mixtures containing fly ash. The sacrificial agent is a material that interacts with components of the fly ash that otherwise neutralize, repress or depress the activity of the air entraining agent component of the cementitious mixture. Representative sacrificial agents include organic compounds having sulfonate, carboxylate or amino functional groups.
Methods and materials that maintain or enhance mercury capture, while lowering foaming index impacts, are thus needed.